This invention relates generally to motor controls and, more particularly, to a control system and method of control for a switched reluctance motor.
Switched reluctance motors conventionally have multiple poles or teeth on both stator and rotor, i.e., they are doubly salient. There are phase windings on the stator but no windings on the rotor. Each pair of diametrically opposite stator poles is connected in series to form one phase of a multi-phase switched reluctance motor. Torque is produced by switching current into each of the phase windings in a predetermined sequence that is synchronized with the angular position of the rotor, so that a magnetic force of attraction results between the rotor and stator poles that are approaching each other. The current is switched off in each phase before the rotor poles nearest the stator poles of the phase rotate past the aligned position. Otherwise, the magnetic force of attraction would produce a negative or braking torque. The torque developed is independent of the direction of current flow so that unidirectional current pulses synchronized with rotor movement can be applied to develop torque in either direction. These pulses are generated by a converter using current switching elements such as thyristors or transistors.
In operation, each time a phase of the switched reluctance motor is switched on by closing a switch in a converter, current flows in the stator winding of that phase, providing energy from a direct current (DC) supply to the motor. The energy drawn from the supply is converted partly into mechanical energy by causing the rotor to rotate toward a minimum reluctance configuration and partly in stored energy associated with the magnetic field. After the switch is opened, part of the stored magnetic energy is converted to mechanical output and part of the energy is returned to the DC source.
U.S. Pat. No. 4,707,650 describes a control system for a switched reluctance motor employing a programmable, closed loop, four quadrant control system incorporating feedback control, angle control and current control. The feedback control incorporates a speed feedback loop and/or a torque feedback loop. The angle control digitally synchronizes stator phase current pulses with rotor position, and the current control acts as a chopping or bang-bang controller to limit the magnitude of the stator phase current pulses. The magnitude and turn-on and turn-off angles of the stator current pulses for each phase, in feedback mode, are controlled so as to provide smooth operation and full torque and speed range with optimum performance in all four quadrants of motor operation, i.e., forward motoring, forward braking, reverse motoring and reverse braking.
The closed loop feedback control processes an actual motor speed signal and an operator command to generate a current command, which serves to limit magnitude of actual phase current, and also generates a turn-on angle signal and a pulse width angle signal which are coordinated with a particular quadrant in which the motor is operating. The values of turn-on angle and pulse width angle are programmable for different quadrants of operation. For motoring quadrants, the turn-on angle signal is directly proportional to the current command while the pulse width angle signal is a function of the current command and actual motor speed.
The digital angle control processes rotor position information signals to generate a multi-phase sync pulse train and individual stator phase signals for the respective stator phases. The angle control also generates a resolution signal with the desired angle resolution. The angle control employs the resolution signal and the individual stator phase sync signals to convert a turn-on angle signal and a pulse width angle signal into corresponding current pulses synchronized with rotor position for each of the stator phases.
The current control compares the current command from the feedback control with actual current in each stator phase to generate a current magnitude limiting signal and couples this signal with the pulse train for each phase from the angle control to generate the stator current control pulses applied to the switching elements in the motor power converter.
While the disclosed system provides for suitable control of a switched reluctance motor, it is believed that further improvement and operation can be attained over a relatively broad low speed range, e.g., for speeds up to about 16,000 rpm, by providing a control system which assures that winding current reaches its commanded set point value at a commanded angle. This desirable feature, which is addressed in one form in U.S. Pat. No. 4,933,620 "Control System for Low Speed Switched Reluctance Motor" by S. R. MacMinn and J. W. Sember issued June 12, 1990, is important to the operation of the switched reluctance motor over a wide speed range because the counter electromotive force (CEMF) in the motor is a function of the angular velocity of the rotor of the motor. For example, with the same set of turn-on and turn-off angles at higher speed, the CEMF is positive in polarity at the beginning of a current pulse, thus opposing the injection of current into the winding, while the end of the current pulse may extend past the alignment position causing the CEMF to become negative in polarity and forcing current to be retained in the winding. The amount of delay in the current pulse reaching its desired value is a function of current level, speed and position of the current pulse. The effect, given a fixed set of turn-on and turn-off angles, is to greatly reduce the amount of motoring torque that can be produced as speed increases thus causing the torque, as a function of current, to become a strong function of motor speed.
As is described above, increasing motor velocity results in generation of a larger CEMF. At some point, the CEMF becomes sufficiently large and limits motor current such that commanded current cannot be achieved. At speeds above this point, some method of control must be used other than current control or motor torque cannot be regulated.